1. Field of the Invention
The present invention relates to a computer control system, and, more particularly, to a hierarchial control system.
2. Description of the Prior Art
In the art of process control instrumentation, various levels of computer based technology have been employed in accomplishing the desired control. At the lower end, there have been provided small microcomputers which are dedicated to the particular control function. At the other end, there are large computers which may be of the general purpose type and which exercise direct control of the ultimate control functions. At the lower end, the dedicated smaller units are limited in capacity, versatility and scope. In the larger units, the direct control of the input/output functions becomes cumbersome.
At an intermediate level, there has been provided a distributed control system wherein a large scale computer comprises the primary system control, while lower level control functions are effected by lower level dedicated control computers. In the known systems heretofore provided, there have been various shortcomings of the intermediate control level which interfaces the main computer with the ultimate control function. In some such apparatus, the intermediate control apparatus has not the capability to gather, store, or manipulate raw data but must transmit that raw data, word by word, to the central processor.
Other such apparatus, while providing for manipulation of the raw data, has no provision for gathering and storing process data, nor can it originate a request for access to the central processor unit. Data is accessed to the central processing unit by directly addressing the ultimate control or sensing function from the CPU.
Although both of the referenced devices represent a significant advance over otherwise extant prior art, there is room for improvement in the area of flexibility, versatility and economy of time and apparatus.
When the central processor is located at, for example, a control room, and the lower level instrumentalities are located at a substantial distance from that control room, interconnecting multiconductor cables become cumbersome and very expensive. Accordingly, it has been found desirable to use a two conductor, preferably coaxial, data bus to interconnect the remote units with the central processor unit. The use of such a two conductor bus, of course, requires that the data on the bus be transmitted in serial form. When the data is transmitted in serial form, it must entail a substantial reduction in the speed of transmitting data to and from the central processor unit with respect to all of the remote data points. Heretofore, in the interest of economy, the speed of transmission has been sacrificed.